1. Field of the Invention
The present invention relates to an acousto-optic filter that utilizes an acousto-optic effect.
2. Description of the Related Art
Recently, data traffic has been rapidly increasing because of the proliferation of the Internet and an additional demand for greatly increasing the capacity for data traffic between in-house LANs. Because of this, optical communication that enables large-capacity communication is becoming widely used from backbone systems to access systems. An increase in optical transmission speed or multiplexing of optical wavelengths can increase the capacity of optical communication. One of the important components for achieving multiplexing of optical wavelengths is an optical wavelength filter. This is a filter that filters light of a predetermined wavelength, and is a key device for performing optical wavelength multiplex communication. In a case in which optical wavelengths are densely multiplexed, when a predetermined wavelength is filtered, it is necessary to block adjacent channels. In order to do so, a necessary filter characteristic of an optical wavelength filter must have low sidelobes with which adjacent channels can be blocked to at least a desired degree.
Acousto-optic filters configured to suppress the above-described sidelobes are described in Japanese Unexamined Patent Application Publication No. 11-64809 and Japanese Unexamined Patent Application Publication No. 8-114776.
FIG. 1 is a plan view showing the structure of an acousto-optic filter described in Japanese Unexamined Patent Application Publication No. 11-64809. In this acousto-optic filter, optical waveguides 111 and 112 that extend in the y-axis direction on an X-cut LiNb03 substrate 110 are provided. On these optical waveguides 111 and 112, a surface acoustic wave (SAW) waveguide 118 defined by an optically transparent thin film whose refractive index is less than that of the substrate 110 is arranged such that the SAW waveguide 118 obliquely crosses the optical waveguides 111 and 112.
At one end portion of the SAW waveguide 118, a transducer 115 including comb-shaped electrodes that generate a surface acoustic wave is provided. Moreover, a SAW absorber 119 defined by a resist is provided at a location so as to sandwich the transducer 115 with the one end portion of the SAW waveguide 118. Moreover, a SAW absorber 120 defined by a resist is provided at the other end portion (a terminal) of the SAW waveguide 118.
In this manner, by arranging a SAW waveguide so as to obliquely cross the optical waveguides 111 and 112, Gaussian distribution weights are assigned in the longitudinal direction of the optical waveguides in TE-TM mode conversion regarding light that propagates in the optical waveguides 111 and 112, and thus, the sidelobes of the filter characteristics are reduced.
FIG. 2 is a plan view showing the structure of an acousto-optic filter described in Japanese Unexamined Patent Application Publication No. 8-114776. In this acousto-optic filter, a transducer 212 for exciting a surface acoustic wave is provided on an x-cut LiNb03 substrate 210. On the substrate 210, a SAW waveguide 216 which is an area divided by two parallel SAW waveguide wall units 214a and 214b is provided, and an optical waveguide 218 is provided along the SAW waveguide 216. SAW absorbers 220a and 220b are each provided at a corresponding one of the end portions of the SAW waveguide.
Moreover, this acousto-optic filter includes an incident light polarizer 222, which is coupled to the optical waveguide 218 and which treats only polarized waves in a specific direction (TE waves) as signal light. Regarding the TE waves isolated by the polarizer 222, only signal light having a specific wavelength according to the wavelength of the SAW is converted into TM waves. As a result, signal light that has been converted into the TM waves whose polarizing direction is orthogonal to the polarizing direction of the TE waves is isolated from outgoing light by an outgoing light polarizer 224.
In this manner, since the two parallel SAW waveguide wall units 214a and 214b that sandwich the optical waveguide 218, which is straight, are arranged in a direction oblique to the direction in which the optical waveguide 218 extends, TE-TM mode conversion can be performed without significantly changing coupling coefficients along the optical waveguide.
In the acousto-optic filter described in Japanese Unexamined Patent Application Publication No. 11-64809, the sidelobes of the filter characteristics can be reduced by assigning Gaussian distribution weights in the longitudinal direction of the optical waveguide in TE-TM mode conversion. However, when an apparatus was actually produced and the characteristics thereof were measured, it was discovered that a value that represents a reducing-sidelobe effect was considerably less than the theoretical value. FIG. 3 shows the characteristics. In this example, light of a wavelength of 1575 nm passes through and light of other wavelengths is blocked. The theoretical value of the maximum sidelobe ratio is −40 dB; however, the maximum sidelobe ratio is practically about −15.6 dB. Even in this case, there is a larger suppressing-sidelobe effect than in the conventional case in which the maximum sidelobe ratio is 90 dB and in which the assignment of weights to the SAW intensity is not performed. However, further suppression of the above-described sidelobes is required in order to improve the degree of optical wavelength multiplexing.